JPH03193490A - Thermal recording material - Google Patents

Abstract

PURPOSE:To efficiently suppress the fading of a developed color image with the passage of time with good reproducibility by constituting a coupler of a mixture of at least one kind of a coupling compound having an m.p. of below specific temp. and at least one kind of a coupling compound having an m.p. of the specific temp. or higher in a molecular level. CONSTITUTION:Bisphenol A being at least one kind of a coupling compound having an m.p. of below 160 deg.C and 1,1-bis(4'-hydroxyphenyl)-1-phenyl-ethane (m.p. ; 188 deg.C) being at least one kind of a coupling compound having an m.p. of 160 deg.C or higher are dissolved in acetone and water is added to the acetone solution under heating to form a white suspension which is, in turn, cooled and subsequently filtered to obtain a coupling compound molecular mixture. This coupling compound molecular mixture, a dye precursor, calcium carbonate pigment, a 30% paraffin solution and a 10% polyvinyl alcohol aqueous solution are mixed and stirred to prepare a coating solution which is, in turn, applied to base paper and dried to obtain thermal recording paper having a thermal color forming layer formed thereto.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a heat-sensitive recording medium, and more specifically, a heat-sensitive recording medium that has excellent color image stability over time, heat resistance, and moisture-resistant storage stability. It's about the body.

[Conventional technology]

Usually, heat-sensitive recording materials that utilize a heated color-developing reaction between a colorless or light-colored dye precursor such as a leuco dye and a color developer such as phenols, particularly bisphenol A, are disclosed in, for example, Japanese Patent Publication No. 43-4160, Japanese Patent Publication No. 45 It was published in Japanese Patent Publication No. 14039, Japanese Patent Publication No. 48-27736, etc., and has been widely put into practical use. Such heat-sensitive recording materials have the advantage that a colored image can be formed by simply heating them, and the recording device can be made relatively compact, so in recent years, they have become popular.
It has come to be widely used as a variety of information recording materials. In particular, thermal facsimile using a thermal recording medium,
BACKGROUND ART The range of use of thermal printers and other devices is rapidly expanding as improvements have progressed and it has become possible to perform high-speed printing and high-speed image formation, which was previously considered difficult. With the expansion of the scope of use, it is desirable to solve the problem of images that are colored by heating becoming discolored over time, and that this discoloration is accelerated under high temperature or high humidity. It is becoming. In order to suppress the decolorization of colored images, the conventional method has been to use antioxidants, especially those with hydroxyl groups, that have been developed and used to prevent the deterioration of rubber and plastics due to oxidation. Hindered phenols, which are compounds with a substituted benzene ring and whose ortho position is further substituted with bulky substituents such as tertiary-butyl, cyclohexyl, phenyl, and adamantyl groups, are heat-sensitive. It is used in combination in the coloring layer. Such technology is disclosed in, for example, Japanese Patent Application Laid-open No. 57-1513.
No. 94, No. 58-160191, No. 58-69096
No. 59-2884, No. 59-95190, No. 6
No. 0-22288, No. 60-255485, No. 61-
No. 44686, No. 62-169683, No. 63-17
No. 081, JP-A-1-249385, JP-A-1-24
This is specifically disclosed in No. 9385 and the like.

[Problem to be solved by the invention]

The above-mentioned hindered phenols, which are used to suppress the decoloring of colored images formed on thermosensitive recording materials over time, do tend to suppress the speed of decoloring when added to the thermosensitive coloring layer. However, adding a small amount is by no means sufficient, and if a large amount is used, there is a problem that the whiteness and sensitivity of the heat-sensitive recording medium will be reduced.
Further, even when the same chemicals are used, there is a problem in that the decolorization inhibiting effect lacks reproducibility, especially when the amount added is small.

An object of the present invention is to provide a heat-sensitive recording material that can efficiently and reproducibly suppress decolorization of a colored image over time.

[Means to solve the problem]

The heat-sensitive recording material of the present invention includes a sheet-like substrate, a colorless or light-colored dye precursor formed on at least one surface of the sheet-like substrate, and a dye precursor that reacts with the dye precursor under heating to develop color. It has a heat-sensitive coloring layer containing a color developer,
The color developer is characterized in that it consists of a mixture at the molecular level of at least one color developer compound having a melting point of less than 160°C and at least one color developer compound having a melting point of 160°C or higher. It is something to do.

In the heat-sensitive recording material of the present invention, the color developer contained in the heat-sensitive color forming layer includes at least one color developer compound having a melting point of less than 160°C (low melting point color developer compound) and a melting point of 160°C or more. By using a mixture at the molecular level with at least one color developing compound (high melting point color developing compound) having It will be done. Further, when bisphenol 8, which has a melting point of less than 160° C., is used as the low melting point color developing compound, the above effect is even more remarkable.

A heat-sensitive recording material obtained by using a mixture of only two types of color developer compounds with a melting point of less than 160°C at a molecular level has a drawback of low whiteness, and a color developer with a melting point of 160°C or more has a disadvantage of low whiteness. The use of a mixture of only two types of compounds at the molecular level shows the drawback that the sensitivity of the resulting heat-sensitive recording material is insufficient.

The mixture at the molecular level of the low melting point color developer compound, especially bisphenol A, and the high melting point color developer compound is:
For example, the mixture obtained by heating and melting a mixture of the two and then lowering the temperature to crystallize, or more practically, dissolving the mixture of both in a solvent that can dissolve it, such as ethanol or acetone, and It means a mixture obtained by adding a solvent in which both are insoluble or poorly soluble, such as water, to a solution and recrystallizing the two. The mixture of the low melting point color developing compound and the high melting point color developing compound obtained in this way can be melted by heating it to above the melting point, even if the temperature of the melt is lowered. It is observed that it takes a very long time to crystallize again. It is thought that this phenomenon is deeply connected to the effect of suppressing the speed of decolorization of a colored image, but the mechanism of suppressing decolorization has not yet been fully clarified.

Examples of the color developing compound having a melting point of less than 160°C include the following. i.e. bisphenol A
(1 point 156°C), benzyl p-hydroxybenzoate (JP-A-52-140483, melting point 110°C), 4-
Hydroxy-4'-isoprobyloxydiphenylsulfone (JP-A-60-13852, melting point 129°C), and 1,7-bis(hydroxyphenylthio)-3,5
-dioxahebutane (JP-A-59-52694, melting point 100°C), etc.

Furthermore, examples of the color developing compound having a melting point of 160° C. or higher include the following.

Namely, bis(4-hydroxyphenyl)methane (bisphenol F, melting point 160°C), 1,1-bis(4'
-hydroxyphenyl)-1-7, enyl ethane (melting point 188°C), i,i-bis(4'-hydroxyphenyl)cyclohexane (melting point 175°C), 4-phenylphenol (melting point 164°C), bis(3', 5'-
dimethyl-4-hydroxyphenyl) sulfone (1 point 2
98°C), bis(4-hydroxyphenyl)sulfone (
4°4'-biphenol (melting point 278°C), 4°4'-biphenol (melting point 278°C)
C), 4-hydroxybiphenyl-3-carboxylic acid (melting point 217C), salicylic acid derivative zinc salt, and zinc oxynaphthoate.

The thermosensitive color forming layer of the thermosensitive recording material of the present invention comprises a dye precursor, a color developer consisting of a mixture at the molecular level of a high melting point color developer compound and a low melting point color developer compound, and, if necessary, an additive. It contains a heat-fusible organic compound known as a sensitizer. Further, the heat-sensitive coloring layer preferably contains an inorganic pigment, and may further contain waxes if necessary.

Furthermore, the thermosensitive color forming layer contains a binder for binding these components and fixing them to the support.

The content of the dye precursor in the thermosensitive coloring layer is generally 5 to 20
% by weight, and the content of the developer mixture is generally between 10 and 4
It is 0% by weight. It is preferable to mix 3 to 100% by weight, particularly preferably 5 to 50% by weight, of a high melting point color developing compound with the low melting point color developing compound (for example, bisphenol A) at the molecular level. If the amount of the high-melting point color developing compound is less than 3%, the effect of inhibiting color fading will not be sufficient, and if it exceeds 100%, the whiteness of the heat-sensitive recording material may decrease.
There are some drawbacks such as decreased sensitivity. The binder content is generally 5 to 20% by weight, and when white pigments and waxes are included, their content is generally 10 to 50% by weight and 5 to 3% by weight, respectively.
It is about 0% by weight.

The dye precursor can be selected from leuco dyes conventionally used in thermosensitive coloring layers, including the following. namely, crystal violet lactone, 3-(N-ethyl-N-isopentylamino)-6
-Methyl-7-anilinofluorane, 3-diethylamine-6-methyl 7-anilinofluorane, 3-diethylamino-6-methyl-7-(o,p-dimethylanilino)fluorane, 3-(N -ethyl-p-toluidino)-6-methyl-7-anilinofluorane, 3-pyrrolidino-6-methyl-7-anilinofluorane, 3-dibutylamino-6-methyl-7-anilinofluorane, 3
-(N-cyclohexyl-N-methylamino)-6-methyl-7-anilinofluorane, 3-diethylamino-
7-(0-chloroanilino)fluoran, 3-diethylamino-7-(m-)lifluoromethylanilino)fluoran 3-diethylamino-6-methyl-7-chlorofluoran, 3-diethylamino-6-methylfluoran, and 3-cyclohexylamino-6-chlorofluorane, and these may be used in combination of two or more thereof, or may be used alone.

The heat-sensitive color forming layer of the present invention may contain a sensitizer as described above, and such a sensitizer preferably includes a group of specific sensitizers having a melting point within the range of 80 to 120°C. Organic compounds are used, a typical example of which is 1-hydroxy-2-naphthoic acid phenyl ester (JP-A-5
7-191089), p-benzylbiphenyl (No. 6
0-82382), benzyl naphthyl ether (No. 5
No. 8-87094), dibenzyl terephthalate (No. 58-98285), benzyl p-benzyloxybenzoate (No. 57-201691), dipheninope ditolyl carbonate (No. 5g-136489), m-terphenyl (No. 57-136489), 57-89994), 1,2-bis(m-
Tolyloxy)ethane (No. 60-56588), 1.
5-bis(p-methoxyphenoxy)-3-oxabencune (No. 62-181183), 1,4-bis(p-
Tolyloxy)benzene (Special [1[63-276186
), and 1,2-diphenoxybenzene (Patent Application No. 1999), and 1,2-diphenoxybenzene
-84788).

Organic or inorganic pigments used in the heat-sensitive coloring layer include calcium carbonate, silica, zinc oxide, titanium oxide, aluminum hydroxide, zinc hydroxide, barium sulfate, clay, talc, and surface-treated calcium carbonate. Examples include inorganic fine powders such as and silica, and organic fine powders such as urea-formalin resin, styrene/methacrylic acid copolymer, and polystyrene resin.

Furthermore, the heat-sensitive coloring layer of the present invention may contain various waxes as required. They can be selected from known ones such as paraffin, amide wax, bisimide wax, and metal salts of higher fatty acids. The binders include polyvinyl alcohols of various molecular weights, starches and their derivatives, cellulose derivatives such as methoxycellulose, carboxymethylcellulose, methylcellulose, and ethylcellulose, sodium polyacrylate, polyvinylpyrrolidone, and acrylic acid amide/acrylic ester copolymers. Coalescence, acrylic amide/acrylic ester/methacrylic acid ternary copolymer, styrene/maleic anhydride copolymer alkali salt, polyacrylamide,
Water-soluble polymer compounds such as sodium alginate, gelatin, casein, etc., as well as polyvinyl acetate, polyurethane, styrene/butadiene copolymer, polyacrylic acid, polyacrylic ester, vinyl chloride/vinyl acetate copolymer, polybutyl Latex such as methacrylate, ethylene/vinyl acetate copolymer, styrene/butadiene/acrylic copolymer, etc. can be used.

The above components are mixed in a predetermined ratio, and the resulting heat-sensitive coloring layer coating liquid is applied to paper, coated paper whose surface is coated with clay plastic, synthetic paper made mainly from plastic materials, and plastic film. It is coated on one side of a sheet-like substrate selected from the following and dried to form a heat-sensitive coloring layer.

The coating amount of the coating liquid is 1 to 10 g/dry state.
It is preferable to make it m', 2 to 7 g /
m'' is particularly preferable.The heat-sensitive recording material of the present invention has a color-developed image that is not only the case where a high melting point color developing compound is not used, but also the case where a hindered phenol is added to improve image storage stability. Even when compared to a thermosensitive recording material having a thermosensitive coloring layer, the layer exhibits less discoloration over time and exhibits excellent recording storage properties even in a humidified atmosphere.

〔Example〕

The present invention will be further explained below with reference to Examples. Unless otherwise specified, "parts" and "%" refer to "parts by weight" and "%".
% by weight.

Preparation Example 1 of Color Developing Compound Mixture 30 g of bisphenol A and 1,1-bis(4'-hydroxyphenyl)-1-phenyl-ethane (melting point 188
℃) was dissolved in 30ml of acetone, and 300ml of water was added while heating. The white suspension formed was filtered after cooling to obtain 32.8 g of a mixture of color developer compound molecules.

Preparation Example 2 of Color Developing Compound Mixture 30 g of bisphenol A and 30 g of 1,1-bis(4'-hydroxyphenyl)cyclohexane (melting point 175°C)
g was dissolved in 30rd acetone, and 300rnl of water was added to this while heating. The white suspension formed was filtered after cooling, yielding 32.8 g of a mixture of color developer compound molecules.

Preparation Example 3 of Color Developing Compound Mixture 30 g of bisphenol A and 3 g of bis(4-hydroxyphenyl)sulfone (melting point 248°C) were mixed into 30 mj!
Dissolve it in acetone and heat it to 300mj!
of water was added. The white suspension formed was filtered after cooling, yielding 32.2 g of a mixture of color developer compound molecules.

Preparation Example 4 of Color Developing Compound Mixture 30 g of 4-hydroxy-4'-isopropyloxydiphenyl sulfone and 3 g of 1,1-bis(4'hydroxyphenyl)-1-phenyl-ethane were dissolved in 30-acetone. After melting, 300ml of water was added while heating. The white suspension formed was filtered after cooling, yielding 32.8 g of a mixture of color developer compound molecules.

Example 1 A thermosensitive recording paper was prepared by the following procedure.

■ Ingredients for preparing dispersion A Amount (parts) 3-(dibutylamino)-620-methyl-7-anilite fluoran polyvinyl alcohol 10% solution 10 water
70 This composition was ground with a sand grinder to an average particle size of 1 p.

■ Ingredients for preparing dispersion B Amount (parts) Color-developing compound molecular mixture (Preparation Example 1) 11 Parabenzylbiphenol 10 Polyvinyl alcohol 10% 1llt 10 Water
70 This composition was ground to an average particle size of 1 mm using a sand grinder.

■ Preparation of thermosensitive coloring layer 40 parts of the above liquid A, 160 parts of liquid B, 4 parts of calcium carbonate pigment
0 parts, 20 parts of a 30% paraffin dispersion, and 180 parts of a 10% polyvinyl alcohol aqueous solution were mixed and stirred to prepare a coating liquid. This coating liquid was coated on a 50 g/m' base paper so that the coated amount after drying was 7.0 g/m'' and dried to form a heat-sensitive coloring layer to obtain a heat-sensitive recording paper.

The heat-sensitive recording paper obtained as described above was treated with a supercalender so that the surface smoothness was 800 to 1200 seconds as measured by a Beck smoothness meter. The samples thus obtained were subjected to measurement of color density and evaluation tests of aging, heat resistance, and moisture resistance of the color development area.

Measurement and recording of color density and blank area density To measure sensitivity, a Toyo Seiki Sosho tilt tester was used, and the heat-sensitive coloring layer was heated at a temperature of 130°C and a pressure of 2.5 kg/ct.
1 for 1QQmsec, and 100m at a temperature of 110℃
The samples were heated for 5 ec, and the color density of each was measured using a Macbeth densitometer RD-914, and the measured value was taken as a value representative of the initial color development ability of the thermosensitive recording paper.

Evaluation of preservability of colored parts/preservability over time 2. The test pieces developed using the above method are left in a living room, and the color density after one week is measured using the Macbeth densitometer mentioned above, and the percentage of that value with respect to the initial color density is determined. The storage rate (%) of the thermal recording paper over time was displayed.

・Heat-resistant storage stability: The test piece colored by the above method was
The paper was left in a dryer adjusted to ℃ for 24 hours, and then the color density was measured using the above-mentioned Macbeth densitometer, and the heat-resistant storage rate (%) of the thermal recording paper was expressed as a percentage of the initial color density.

・Moisture resistance storage property: The test piece colored by the above method was
36°C in a Tabai environmental tester adjusted to 90% R11.
After leaving it for a while, the color density was measured using the Macbeth densitometer mentioned above, and the heat-resistant storage rate (%) of the thermosensitive recording paper was expressed as a percentage of the initial color density.

The test results are shown in Table 1.

Example 2 ■ Preparation of pigmented undercoated paper 85 parts of Baked Ray (trade name: Ansilex) were mixed with 32
The resulting dispersion was mixed with 40 parts of a styrene-butadiene copolymer emulsion (solid content 50%) and 50 parts of a 10% oxidized starch aqueous solution. The obtained coating liquid was applied onto one side of base paper with a basis weight of 48 g/m' so that the coating amount after drying was 7.0 g/m', and dried.
A pigment-primed paper was created.

■ Formation of thermosensitive coloring layer 50 parts of dispersion A, 200 parts of dispersion B, 2 parts of calcium carbonate
5 parts of a 30% paraffin dispersion, and 180 parts of a 10% polyvinyl alcohol aqueous solution were mixed and stirred to prepare a coating liquid. The obtained coating liquid is applied onto the pigment-coated surface of the pigment undercoated paper so that the coating amount after drying is 5.0 g/m' and dried to form a heat-sensitive coloring layer, and then the heat-sensitive recording paper is coated with the coating solution. I got it. This thermosensitive recording paper was subjected to the test described in Example 1. The results are shown in Table 1.

Example 3 The same operation as in Example 2 was performed. However, in preparing Solution B, the color developer compound molecule mixture of Preparation Example 2 was used instead of the color developer compound molecule mixture of Preparation Example 1. The test results are shown in Table 1.

Example 4 The same operation as in Example 2 was performed. However, in the preparation of Solution B, the color developer compound molecule mixture of Preparation Example 3 was used instead of the molecule mixture of Preparation Example 1. The test results are shown in Table 1.

Example 5 The same operation as in Example 2 was performed. However, in the preparation of Solution B, the color developer compound molecule mixture of Preparation Example 4 was used instead of the molecule mixture of Preparation Example 1. Of the tests described in Example 1, only the color development test and the aging test were performed on the obtained thermosensitive recording paper.

The results are shown in Table 1.

Example 6 The same operation as in Example 2 was performed. However, in preparing Solution A, 3-(N-isopentyl-N
-ethylamino)-6-methyl-7-anilite fluoran was used. The test results are shown in Table 1.

Comparative Example 1 The same operation as in Example 2 was performed. However, in preparing Solution B, a dispersion of 10 parts of bisphenol A was used instead of 11 parts of the molecular mixture in Preparation Mixing Example 1. The test results are shown in Table 1.

Comparative Example 2 The same operation as in Example 2 was performed. However, in the preparation of Solution B, 10 parts of bisphenol A was used instead of 11 parts of the molecular mixture in Preparation Example 1, and in the preparation of the coating solution when forming the heat-sensitive coloring layer, 1, which had been separately dispersed with an average particle size of 1 m or less, was used.
A further 8 parts of 1-bis(4'-hydroxyphenyl)-1-phenyl-ethane (25% by weight dispersion) were added.

The test results are shown in Table 1.

Comparative Example 3 The same operation as in Comparative Example 1 was performed. However, in the preparation of Solution A, 3-(N-isopentyl-N
-ethylamino)-6-methyl-7-anilite fluoran was used. The test results are shown in Table 1.

As is clear from Table 1, Examples 1 to 6 according to the present invention
All of the thermal recording papers exhibited a practically sufficient color density and better storage rates over time, heat resistance, and stain resistance than the thermal recording papers of Comparative Examples 1 to 3.

〔Effect of the invention〕

The heat-sensitive recording paper of the present invention contains a color developer consisting of a mixture at the molecular level of a low-melting point color-developing compound and a high-melting point color-developing compound in the heat-sensitive coloring layer. , storage stability under heating and humidification can be greatly improved, and is extremely useful in practice.

Claims (1)

[Claims] 1. A sheet-like substrate; a colorless or light-colored dye precursor formed on at least one surface of the sheet-like substrate;
a thermosensitive coloring layer comprising this dye precursor and a color developer that reacts with the dye precursor to develop color under heating, the color developer being at least one color developer compound having a melting point of less than 160°C. and at least one color developing compound having a melting point of 160° C. or higher at a molecular level.